1. Field of the Invention
The present invention relates to a method and system for controlling resistance welding, and more specifically to a method and system of controlling the pressure applied to the electrodes or the current passed therethrough by detecting the resistance between the electrodes or the voltage therebetween, respectively, in order to obtain a stable quality during the welding process.
2. Description of the Prior Art
In conventional resistance welding, for instance spot welding, generally the welding conditions between two members to be welded are largely dependent upon the electrode pressure, the sizes and shapes of the electrodes, the condition of the electrodes, and the kinds or worked conditions of the members to be welded. Therefore, it is impossible to attain a stable welding quality even if the electrode pressure, welding current, and welding time are all maintained at constant levels. In this case, the term "electrode pressure" refers to the pressure applied between the electrodes pinching together two or more members to be welded.
To overcome this problem, conventionally there have been devised various testing systems for monitoring the electrode voltage, the electrode resistance and so on, and other systems for monitoring the welds by means of ultrasonic waves. These monitoring methods or apparatuses, although being applied in various fields, can only determine whether or not the quality of the welded portions is satisfactory after the welding process has been completed, and therefore cannot ensure the quality of the welding during the welding process.
In this case, the term "quality of weld" implies the size and penetration of the beads formed at the welds, the tensile and shear strengths of the welds, and so on.
Therefore, even when the various conventional methods or systems for resistance welding are used separately or together, the welding quality is poor; and it is necessary to correct or repair the weld, or to discard the parts or weldments in the case where the weld quality is beyond repair.
The inventors have already proposed a method of controlling resistance welding in such a manner that the quality of resistance welding can be ensured automatically during the welding process by controlling the electrode pressure or both the electrode pressure and the welding current so that the electrode voltage changes in accordance with a reference voltage curve predetermined to ensure a desirable welding quality. This idea depends upon the fact that the welding voltage applied between two electrodes between which the members to be welded are pinched (including the electrode tips in the case of spot welding) has a close relationship to the quality of the welding.
However, it is impossible to control the size of a bead formed at the weld under optimum conditions at all times depending only upon the method described above. For instance, when the electrode tip is crushed, the diameter of the bead becomes excessively large, thus resulting in excessive power consumption and producing an unnecessarily good quality. On the other hand, there sometimes arises another problem such that it is impossible to obtain the necessary diameter of the bead, even when the elctrode voltage is altered according to the reference voltage curve. This is because the area through which the welding current is passed is not sufficient because of the surface conditions between the members to be welded together.
In order to perform such a resistance welding control, it is necessary to predetermine the reference welding current so that the resistance and voltage between the electrodes pinching the members to be welded together roughly coincides with the predetermined reference values, to control the welding pressure applied to the electrodes in accordance with the difference in resistance between the detected value and the reference value, and also to correct the welding current in accordance with the difference in voltage between the detected value and the reference value.
However, when the power supply voltage fluctuates, since the reference welding current also fluctuates even if the other conditions are kept constant, the control characteristics will deteriorate. Especially when the power supply voltage fluctuates greatly, the response in control of the electrode voltage is slow, and therefore it is impossible to obtain the desired results.